Direct Visualization of CH₄/CO₂ Hydrate Phase Transitions in Sandstone Pores

Abstract

This paper reports the formation and dissociation pattern of hydrate crystals with varying compositions of CH4 and CO2 in porous media. Direct visualization was carried out using a high-pressure, water-wet, silicon wafer-based micromodel with a pore network resembling sandstone rock. Hydrate crystals were formed under reservoir conditions (P = 45–65 bar and T = 1.7–3.5 °C) from either a two-phase system consisting of liquid water and a CH4–CO2 gas mixture or a three-phase system consisting of liquid water, CH4-rich gas, and CO2-rich liquid. A stepwise pressure reduction method was later applied to visualize multiple dissociation events occurring between the equilibrium pressures of pure CH4 hydrates and pure CO2 hydrates. The results showed that liberated gas from the initial dissociation became trapped and immobilized by surrounding undissociated hydrate crystals when the initial hydrate saturation was high. Mixing of liberated gas with liquid water led to rapid reformation of hydrates during the stepwise pressure reduction; the reformed hydrate crystals dissociated at a lower pressure close to the equilibrium pressure of pure CO2 hydrates. The results demonstrate the possibility of producing gas liberated from local hydrate dissociation while simultaneously reforming hydrates in other parts of the sediments. This is relevant for the proposed production method where CO2 injection in CH4 hydrate reservoirs is followed by pressure depletion to enhance the CH4 gas recovery.

Cyclic Depressurization-Driven Enhanced CH4 Recovery after CH4–CO2 Hydrate Swapping

“…Further depressurization near the CO 2 stability pressure would lead to dissociation of the unstable mixed hydrate because the driving force is no longer sufficient, and the increase in the released water will not participate in the reformation process. 38…”

“…As the pressure inside the vessel moves through the different regions of hydrate stability, phase changes may occur, including dissociation and reformation of hydrates, which would lead to fluid migration and fluid redistribution, thus affecting the yield and storage potential. 38 Previous studies that focused on examining the pressure response curve during the shut-in period suggest three characteristic pressure responses during the degassing. 14 The first is a stable pressure curve during gas expansion; the second is an abrupt pressure drop followed by a rapid pressure rise at the onset of hydrate dissociation; and the third is a stable pressure after complete hydrate dissociation.…”

“…In another study using a high-pressure micromodel, the cyclic dissociation of the mixed CH 4 /CO 2 hydrates showed CH 4 dissociation and simultaneous CO 2 reformation. 38 There are no previous kinetic or morphological data on the dissociation kinetics of the mixed CH 4 /CO 2 hydrate system under multistep cyclic depressurization. Therefore, further studies on different scales and aspects are needed to confirm the application of this technique to enhance CO 2 storage and recover additional CH 4 .…”

Chemically Influenced Self-Preservation Kinetics of CH4 Hydrates below the Sub-Zero Temperature

“…The hydrate morphology controls the total hydrate surface area, while the pore size distribution controls the hydrate stability. The pore-scale visualization studies have shown that hydrate remained stable in narrow pore throats outside of their stability zone [6,66]. In another study, the correlation of the size of the hydrate particles with self-preservation was investigated and the particle particles larger than 0.5 mm in diameter showed a high degree of self-preservation [67].…”

Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

“…The presence of additives and substrate wettability also play a role in hydrate growth [38][39][40]. The most complex configurations observed so far by optical microscopy are those obtained in etched twodimensional silica glass or silicon wafer micromodels [50][51][52] or by using lab-on-a-chip technology [53][54][55].…”